In captivity it is a different story, for you provide the diet, not the wild. Black Walnuts have about an (8:1) ratio of Phosphorus to Calcium. By comparison, the English (Persian) walnut has a (3.5:1) Phosphorus to Calcium ratio.
https://tools.myfooddata.com/nutriti...t6-wt3&qty=1-1
Black walnuts are also higher in oxalates, a calcium robbing anti-nutrient in plant sources. This makes the actual ratio of Calcium to Phosphorus much higher; for the higher the ratio of phosphorus to calcium, the less calcium is absorbed into the bloodstream. Calcium from this nut is nil. Again, by comparison, the English walnut according to a study done with US on oxalates in nuts, though most nuts have around 200 mg. of oxalates, English walnuts have about 74 mg. oxalates, making them one of the lowest in oxalates of all nuts. The Black walnut by comparison is very high in oxalates, leaving no bioavailable calcium to the body. .